Why are not class parameters not being called on the Object in Scala?

Question

In Scala how are not class parameters not truly the values being called on the object? The class parameters are part of primary constructor, so should it not be setting up the member variables of the objects, like Java? Why must we make separate fields that are the values being set on the objects? Rather than just accepting the fact, is there any good explanation?

Could you please provide a small code example to show what you mean? If I understand your question, you want to turn parameters to primary constructors into fields, which you can do by putting a `var` or `val` before the parameters. — user, Jun 03 '20 at 18:46
class Rational(n: Int, d: Int) { def add(that: Rational): Rational = new Rational( n * that.n + that.d * d, d * that.d) } I understand this is improper and will cause a compile time error, but conceptually I do not get quite. Are not class parameters members of the class, can we not cal it on the object or argument that takes the object? Sorry, for formatting, I was typing on phone. — VividMan, Jun 03 '20 at 18:54
In Java, constructor parameters are also not members, so it's actually exactly the same between the two languages. — Jörg W Mittag, Jun 03 '20 at 19:49

score 5 · Answer 1 · answered Jun 03 '20 at 18:54

5

If you want to call the parameters assigned in class's primary constructor, you will have to declare them as field.

class Man(name: String, age: Int) {
      def show = "My name is " + name + ", age " + age
}

Here name and age is constructor parameter. They are only accessible in the class scope.

class Man(val name: String, age: Int) {
      def show = "My name is " + name + ", age " + age
}

Notice name is now a variable. So now you can access name with Man class's instance.

val x  = new Man("Jack",10)
x.name // Jack

But you can not access age because it is a parameter not field.

answered Jun 03 '20 at 18:54

John

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Or even better use **case classes**. – Luis Miguel Mejía Suárez Jun 03 '20 at 19:11
@LuisMiguelMejíaSuárez case class has use case. Case class is used for data modeling class. – John Jun 04 '20 at 11:13

Mario Galic · Answer 2 · 2020-06-03T19:58:05.523

The class parameters are part of primary constructor, so should it not be setting up the member variables of the objects, like Java?

Even in Java constructor parameters do not automatically become class members

public class User {
    public User(String name, Integer age) {
        // do something
    }
}

User user = new User("Picard", 75);
String name = user.name // error

so we have to do something like

public class User {
    String name;
    Integer age;
    public User(String name, Integer age) {
        this.name = name;
        this.age = age;
    }
}

Why must we make separate fields that are the values being set on the objects?

Using case class we get that for free (along with few other things)

case class User(name: String, age: Int)
val user = User("Picard", 75)
val name = user.name // ok

score 0 · Answer 3 · answered Jan 16 '22 at 09:53

In Scala how are not class parameters not truly the values being called on the object?

Indeed, in certain cases class parameters are implemented as fields with the scope set to private[this].

Here is an excerpt from the discussion in scala-lang website.

A parameter such as class Foo(x : Int) is turned into a field if it is referenced in one or more methods
Such a field is as if it had been declared as private[this] val x: Int.
If the parameter isn't referenced in any method, it does not give rise to a field.

Why are not class parameters not being called on the Object in Scala?

3 Answers3